Rewrite your manuscript or nonfiction proposal until it's ready. You never want to send out a first draft. Novels typically must be complete works, while nonfiction proposals typically require two or three chapters and an outline. Because in all likelihood you only get one chance to impress the agent or editor, take your time and make your work the best it can be.
Research agents and small presses online, and from books listing literary agents. Look to see which places are looking for works similar to yours. You don't want to send you romance novel to an agent who's only looking for memoirs. If your goal is to place a book with a big publishing house, you probably need to go through an agent. Big publishing houses don't accept unsolicited material, so target agents who represent works like yours.
Write a query letter to the agent or small press. This should be a page long and be specific to the agent or editor. If you met the person at a writer's conference or received a recommendation from someone they know, mention it in the first paragraph. Briefly note any short stories or articles of yours that have been published. Also mention any awards for writing.
Nonfiction writers should mention if they have an online audience and their platforms. For example, if you're writing about the education system in South Carolina, you need to establish why you have the expertise to write the book. Some agents and small presses might want you to include the first five to 10 pages of the book with the query. Each agent and small press is different, so follow the submission guidelines on their website.
Send the query and a self-addressed stamped envelope for an agent or editor to respond to your request. Response times can take six months for some agents and editors. If they ask you for a full or partial manuscript, thank them and send the material. You'll have to wait again for a response, so be patient. Unless the agent or editor has as for an exclusive, it's typically acceptable to send out multiple submissions.